I finally got the chance this week to do the planned work. I replaced the pistons in seals in the remaining two calipers and installed stainless lines front and rear. Then I pressure bled all four calipers twice, going through roughly 1.25 liters of brake fluid in total. The rubber lines I removed all seemed fine, with no cracks or leaks. I did find a bit of jellied brake fluid between the wiper seal and fluid seal on one of the pistons in the rear caliper, indicating a minor leak.

The results of all the repairs was an increase in actually braking power, but no change in the performance of the pedal. I can still drive it to the floor under constant pressure and stopping power takes much longer pedal travel than it should. Pumping up the pedal does nothing to increase the pedal firmness, indicating that the system is clear of air.

I did a short road test where I took the temperature off all four rotors prior to driving and then again afterwards, with frequent braking. Prior to the drive, all rotors read at 67 degrees. After, both fronts were around 200 degrees. The right rear was 120 and the left rear 90. I'll retest tomorrow to see if I get the same discrepancy in the rear. If so, I guess it could mean an ABS module failure that's relieving pressure from that caliper? Although I don't feel the ABS engaging.

What about bearings? If rear bearings are worn, you can't feel it depressing wheel to feel it because of the shafts. But this small amount of movement necessary to accomodate disc can take a lot of pedal distance (and brake pads from a rear wheel wear unequal).

What about bearings? If rear bearings are worn, you can't feel it depressing wheel to feel it because of the shafts. But this small amount of movement necessary to accomodate disc can take a lot of pedal distance (and brake pads from a rear wheel wear unequal).

Regards

I thought of that. Here my reasons for dismissing it, at least for the moment. Let me know if my reasoning is flawed and I've missed something.

1) I don't have zero brakes for a portion of the pedal travel and then achieve normal brakes later in the travel. In other words, it's not the "OH crap, no brakes!" that you feel with pad knock back and then you find them after pumping the pedal. Light pressure on the pedal yields stopping power and there is stopping power spread over the entire range of the pedal movement. That movement is just elongated from normal and with reduced overall power for hard stopping. Granted, I would still get this with one or even two bearings causing those pads only to be pushed back.
2) However, I would expect a brake pull in one direction or another with this cause, as a uniform failure of the bearings seems unlikely. However, I don't have that.
3) I don't get the pedal back by pumping it, which would in theory move the pads back up against the rotors after being pushed back by bearing wobble. I'm able to move the pedal to the floor at a standstill repeatedly after pumping it up in between.
4) I just replaced the stub axle seal on both rear wheels and checked the bearings while I had those hubs apart, all felt good.
5) I'll recheck each wheel for play today, as I did that way back at the start of this process, but I don't recall anything out of the ordinary.

Thanks very much for the suggestion and let me know if you think I should rethink it.

If you have even slightly worn bearings you get wheel wobble. Wheel wobble pushes the caliper pistons out.

To test it start the truck and press the pedal until it?s firm consistently.

Now drive. If your pedal then goes to the floor or longer travel one of your wheel bearings is probably loose.

If your pedal will never firm up, even when sitting still and pumping it (pump it a ton of times to get the pistons extended in case the above is occurring), then I'd look to the power booster thing.

I chased all kinds of problems with no leaking fluid and finally bought a new MC and Servo. Replacing both was a cake walk.

Thanks for the suggestions. You are correct, I never get a firm pedal at standstill. I can pump it up to feeling initially firm after 4 to 5 long, slow pumps, but then it will just descend under constant pressure. I agree that this would normally leave the MC or the booster. However, I ruled those out earlier by blocking off the master cylinder at the outlets and then testing the pedal with the car both on and off. In that case I had a firm pedal. I'm also clearly getting brake boost and the booster is holding vacuum when tested at the end of the tube running to the engine. So, it would appear to be an issue further down the line from the MC/Booster. Even though RAVE lists the MC and Booster as the only two possible causes of a descending pedal, I don't see that those are my issues, based on the symptoms and tests I've run.

If I've missed something in my thinking, please let me know. The more suggestions I get the more it helps me. Thanks everyone.

However, I ruled those out earlier by blocking off the master cylinder at the outlets and then testing the pedal with the car both on and off. In that case I had a firm pedal.

I'm also clearly getting brake boost and the booster is holding vacuum when tested at the end of the tube running to the engine. So, it would appear to be an issue further down the line from the MC/Booster. Even though RAVE lists the MC and Booster as the only two possible causes of a descending pedal, I don't see that those are my issues, based on the symptoms and tests I've run.

Maybe they work well enough to hold when you have the MC blocked off but not well enough when you have the entire system in play? I have no idea if that even makes sense in physics world, but kind of does in my brain.

Sometimes I get to a point where throwing parts at a problem starts to make sense, especially if the parts I'm replacing are already 20 years old....

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Maybe they work well enough to hold when you have the MC blocked off but not well enough when you have the entire system in play? I have no idea if that even makes sense in physics world, but kind of does in my brain.

Sometimes I get to a point where throwing parts at a problem starts to make sense, especially if the parts I'm replacing are already 20 years old....

Point taken and I agree with you on the fact that at some point, you just have to eliminate as best as possible and try parts. I'm just trying to pick my financial battles wisely on this car. Believe me, I'd love to just start overhauling with the idea that if it doesn't fully solve the issue, at least it's a new part. I didn't expect the stainless lines to solve anything, but I new those would not only replace a part that was recommended to be replaced many miles ago, but would also be an improvement over the original.

My frustration and hesitation to replace the booster is that it passes every test, without any indication of failure to any degree:

~With the vacuum line disconnected, air doesn't come out when pressing on the pedal. Indicating the check valve is operating correctly.
~Holding the brake pedal with the car running, then shutting the car off and slowly pressing the pedal three times results in a gradually higher and higher pedal. Also, running the engine and pressing and holding the brake pedal, then shutting of the engine while maintaining pressure on the pedal doesn't result in a drop of the pedal once the engine is off. Both indicate the internal seals are airtight.
~Lastly, maintaining pressure on the brake pedal with the car off and then starting the engine results in a slight drop in the pedal, indicating the booster is providing proper assist. The only issue is that in my case the pedal continues to descend under constant pressure.

I've read that a bad ABS modulator can mimic the symptoms of a bad master cylinder. Essentially the valves which normally would release pressure from a caliper who's wheel is skidding are not fully closed under normal driving, creating the same pass through experienced with a bad MC. Anyone ever had this? Any way to test for it?

Air can get trapped in strange places. Have someone gently put pressure on the pedal and crack lines loose at the master cylinder. See if you get any air up there. Gently is the key word there. You could spray brake fluid to a painted surface.

Point taken and I agree with you on the fact that at some point, you just have to eliminate as best as possible and try parts. I'm just trying to pick my financial battles wisely on this car. Believe me, I'd love to just start overhauling with the idea that if it doesn't fully solve the issue, at least it's a new part. I didn't expect the stainless lines to solve anything, but I new those would not only replace a part that was recommended to be replaced many miles ago, but would also be an improvement over the original.

My frustration and hesitation to replace the booster is that it passes every test, without any indication of failure to any degree:

~With the vacuum line disconnected, air doesn't come out when pressing on the pedal. Indicating the check valve is operating correctly.
~Holding the brake pedal with the car running, then shutting the car off and slowly pressing the pedal three times results in a gradually higher and higher pedal. Also, running the engine and pressing and holding the brake pedal, then shutting of the engine while maintaining pressure on the pedal doesn't result in a drop of the pedal once the engine is off. Both indicate the internal seals are airtight.
~Lastly, maintaining pressure on the brake pedal with the car off and then starting the engine results in a slight drop in the pedal, indicating the booster is providing proper assist. The only issue is that in my case the pedal continues to descend under constant pressure.

I've read that a bad ABS modulator can mimic the symptoms of a bad master cylinder. Essentially the valves which normally would release pressure from a caliper who's wheel is skidding are not fully closed under normal driving, creating the same pass through experienced with a bad MC. Anyone ever had this? Any way to test for it?

I think what you might be talking about is an unwanted abs activation. That's usually caused by a loose wheel bearing but could be a wheel speed sensor too. It makes you feel like your going to glide right into something at slow speeds. But when you start to freak out and press the pedal harder the abs works and can grind it to a stop. I've heard crazy talk of taking out the whole abs system for that. Tightening up the wheel bearing and pushing the sensor back in is probably easier.